1. Field of the Invention
This invention relates to a process for producing a porcelain consisting of a solid electrolyte, specifically a .beta.-alumina porcelain, which has high sodium ion conductivity and good stability.
2. Description of the Prior Art
A .beta.-alumina porcelain which is regarded as a commercially prospective sodium ion conductor is a double oxide expressed by the chemical formula Na.sub.2 O.xAl.sub.2 O.sub.3. It includes two types, .beta.-Al.sub.2 O.sub.3 (where x=9-11), and .beta."-Al.sub.2 O.sub.3 (where x=5-7).
For the purpose of the present description, both .beta.-Al.sub.2 O.sub.3 and .beta."-Al.sub.2 O.sub.3 will be referred to herein generically as .beta.-alumina, whereas the designation .beta.-Al.sub.2 O.sub.3 will be only used to denote specifically Na.sub.2 O.xAl.sub.2 O.sub.3 wherein x is 9 to 11.
.beta."-Al.sub.2 O.sub.3 has a high sodium ion concentration and an ion conductivity several times higher than that of .beta.-Al.sub.2 O.sub.3, but the stability of .beta."-Al.sub.2 O.sub.3 is poor. When .beta."-Al.sub.2 O.sub.3 is heated to a temperature of more than 1550.degree. C, it is transformed into .beta.-Al.sub.2 O.sub.3. At room temperature, (about 20.degree.-30.degree. C), too, .beta."-Al.sub.2 O.sub.3 reacts with CO.sub.2 or moisture in the air, and as a result the ion conductivity of .beta."-Al.sub.2 O.sub.3 decreases. In extreme cases, .beta."-Al.sub.2 O.sub.3 tends to disintegrate. Thus, in spite of many suggestions for commercialization, no practical applications for .beta."-Al.sub.2 O.sub.3 have been found. In contrast, .beta.-Al.sub.2 O.sub.3 has good stability, but the insufficient ion-conductivity of .beta.-Al.sub.2 O.sub.3 has prevented practical application thereof.